Portable radios crackle. Not to mention they broadcast on the open air waves. Slipping a note is dangerous. Mobile phones donít cut it. And a shout would ruin everything. So how do you whisper a message to a colleague without being noticed?

How do you pass on a message while in a crowded room, without blowing your cover?

How do you listen to music while studying under the watchful eyes of your parents?

The Massachusetts Institute of Technology (MIT) have found a way.

Theyíre beaming lasers into peopleís ears.

There are no specialised earpieces receiving the signal.

Just the original, naked Mark I ear canal.

And itís all because of something called the photoacoustic effect.

The invisible laser interacts with its environment.

In particular, moisture.

Where the laser strikes moisture in the air, its light is absorbed and transformed into sound waves. And sweeping the laser at different lengths at the speed of sound can be used to encode different audio frequencies.

ďOur system can be used from some distance away to beam information directly to someoneís ear,Ē project leader Charles Wynn says.

Lasers have been used before, to decode the vibrations of voices as they strike surfaces such as glass.

But this transmits, silently.

Itís something that will whisper through the still air of a quiet night. Itís also something that can deliver a targeted message in a crowded room, without anybody overhearing.

And the effect only works at pre-programmed distances.

Someone walking across the beam of invisible light will hear nothing.

But will the laser fry your hearing?

No, the MIT team insists. The laser system being used cannot harm skin or eyes. And the sound that tickles your inner-ear follicles is the same as any other.

Itís just that it comes from vibrating moisture only your ears can detect.

ďThis can work even in relatively dry conditions because there is almost always a little water in the air, especially around people,Ē Wynn says.

So far, laboratory experiments have succeeded in transmitting clearly audible (60 decibel) messages over some 2.5 meters. They say the technology can be easily scaled to cover greater distances.

ďWe hope that this will eventually become a commercial technology,Ē says the lead author of the study published in the journal Optics Letters, Ryan Sullenberger. ďThere are a lot of exciting possibilities, and we want to develop the communication technology in ways that are useful.Ē